1. Field of the Invention
The present invention is directed to a sum current transformer having a wound, lapped core of high-permeability, soft-magnetic material for acquiring the current of current conductors passing through the core, whereby the winding of the core is connected via an amplifier to a protective switch.
2. Description of the Prior Art
A current transformer of the above general type is disclosed in PCT Application WO 93/16479. The core of this known sum current transformer can be optionally composed of sintered, ferromagnetic material, or disks stacked on top of one another, or wound bands or wires. All of these core versions have in common the fact that insulator layers are provided by small air gaps in the material or by the division into disks or by winding, these insulator layers reducing eddy currents induced in the material by the alternating field acting thereon. The result is that such cores--especially because of their small dimensions--have low mechanical strength and are therefore sensitive to shock stresses and also have low strength for being wound.
When a sum current transformer is connected to the input of an amplifier, i.e. the power for the switching of a relay is not taken from the core itself, then it requires a relatively low transmission power and can therefore be implemented with small dimensions. The miniaturization of the dimensions is essentially limited by the mechanical weakening of the core of the sum current transformer associated with the dimension reduction and by the unavoidable increase in the ohmic resistance of the winding, since this must then be composed of relatively thin wires. This ohmic resistance of the winding of the core of the sum current transformer, however, is the determining factor for the gain of the following amplifier--among other things. Since the ohmic resistance changes with the temperature, the amplifier will also exhibit a temperature response, so that the precision of the trigger characteristic will suffer therefrom.